As leaf-harvest plants, tea trees show unique nutrient requirements, different from those of corn and other field crops. However, the effects of nitrogen (N), phosphorus (P), and potassium (K) application on the accumulation of quality-related compounds and the mechanisms underlying how nutrients affect tea-leaf metabolism have not been well elucidated. Here, fertilizers with different N, P, K ratios were applied to tea plants in pot experiments, and metabolomics based on gas chromatography-mass spectrometry (GC-MS) combined with multivariate statistical and quantitative detections were conducted to assess the responses of quality-related compounds to NPK in tea leaves. An increased proportion of P and K was beneficial for the accumulation of carbohydrates and catechins in shoots, although the total carbon content did not increase significantly. In contrast, a high proportion of P and K input reduced the relative chlorophyll content in shoots, and the contents of free amino acids such as theanine and glutamic acid negatively correlated with P and K nutrient content. Moreover, the metabolism of malic acid in the tricarboxylic acid cycle was highly promoted by increasing the application of P and K. These results validate our suggestion that the application of high amounts of P and K in tea plantations induces the biased reallocation of photosynthates and carbohydrates to the catechin pathway by promoting malic acid metabolism in young tea shoots, which further affects tea quality. The results of this study provide theoretical ground for tea quality improvement by optimizing fertilization strategies.

A moderate supply of aluminum (Al) can promote the root growth of plant species, such as tea, that have adapted to acid soils. The formation and development of roots are regulated by phytohormones. However, the role that phytohormones play in facilitating the beneficial effects of Al on root growth is unclear. In the present study, tea plants cultured hydroponically with either 0 or 0.4 mmol·L−1 Al were characterized and compared in terms of root growth, hormone concentrations, and the expression of related genes. The hormone content in tea roots was measured by UPLC–MS, and the expression levels of genes involved in phytohormone biosynthesis, transport, and response were analyzed by qRT-PCR. Our results showed that the indole acetic acid (IAA) concentrations in the root base were significantly higher in plants supplied with Al compared to control plants without Al during the experimental period from 6 h to 7 d. By using inhibitors of IAA synthesis and transport, we found that Al induces short-term IAA synthesis in the root tip and enhances IAA transport in other parts of the root over the longer term, thus promoting root growth in tea plants by limiting IAA accumulation. The expression levels of genes YUC8 and AMI1, involved in IAA accumulation, were consistently upregulated in the root tip, whereas expression of PIN1/2/8, involved in IAA transport, was induced in the root base under Al supply at 6 h. In addition, the expression of PIN2 and PIN5 was highly induced in root stem by Al after 7 d and probably contributed to alleviating IAA accumulation in the root tip and thereby promoted root growth in tea plants. Meanwhile, the XET and expansin activities were increased under Al supply at 6 h. Thus, we conclude that the mechanism of Al-induced root growth is mediated by auxin accumulation and transport in cooperation with cell wall loosening in tea plants.